An inverter according to the present invention may also be used for other applications than feeding electric energy in an AC power grid. For example, it may provide an output AC voltage of varying frequency to control an electric AC motor, for example.
Further, the inverter according to the present invention may either output a single-phase AC voltage or a multi-phase AC voltage, particularly a three-phase AC voltage. When, in the following, a particular inverter is described as being a single-phase or a three-phase inverter, this description should always be read to also cover corresponding multi-phase or single-phase inverters.
Even further, a combination of the inverter according to the present invention with further electric or electronic units, like, for example, a boost-converter at its input side, is possible and may be useful. The present invention relates to the construction of the inverter as such.
German patent DE 10 2004 030 912 B3 discloses an inverter with a H-shaped inverter bridge between two input lines. The inverter bridge comprises gate-controlled semiconductor switches of metal oxide semiconductor field effect transistor (MOSFET) construction. These MOSFETs are normally non-conductive, i.e. without a control voltage applied to their gates they are not conductive between their source and their drain. Such semiconductor switches are also described as being of the “normally off type”. Due to this property of its semiconductor switches the overall inverter bridge is non-conductive as long as no control voltage is present at the gates of the semiconductor switches. As a result, a short circuit between the input lines and between output ports of the inverter is automatically avoided in the case of no controller being active. This case of no controller being active occurs, for example, upon using the known inverter for feeding electric energy from a photovoltaic power generator into an AC power grid during the night, and particularly in the morning, when the photovoltaic power generator builds up an output voltage which is not yet high enough to power the controller of the inverter.
The quality of an inverter may be determined by its efficiency factor which strongly depends on the electrical properties of the gate-controlled semiconductor switches used. It is generally known that silicon carbide (SiC) semiconductor switches have particular advantages with regard to the efficiency factor. In addition to low ohmic and switching resistivities, SiC semiconductor switches also have the advantage of being able shut off particularly high voltages. Further, SiC semiconductor switches function properly even at high temperatures of up to 600° C. However, SiC semiconductor switches at reasonable cost are mainly available as JFETs (Junction Field Effect Transistors). In contrast to MOSFETs, JFETs are normally conductive between their source and their drain, if no control voltage is present at their gate, i. e. they are of the “normally on type”. Normally conductive semiconductor switches, however, have restrictions as their use may result in short circuits, if no control voltage is present. This particularly applies to their use in inverters.
To compensate for a JFET normally being conductive, it is known to combine it in a so-called cascode circuitry with a MOSFET which is connected between the gate and the source of the JFET and which is controlled by a control voltage to thus indirectly control the JFET. A cascode, however, has a reduced efficiency and comprises one additional MOSFET per JFET.
US-patent application publication 2008/0174184 A1 discloses a device for controlling a JFET power electronic switch of the normally on type. The control device comprises a main gate-control circuit powered by a main power source for driving the gate of the JFET switch. The control device further comprises an auxiliary power source whose positive terminal is connected to the source of the JFET switch and whose negative terminal is connected to the gate of the JFET switch. The control device comprises a protection switching device that is switchable between two states, an auxiliary circuit for controlling the switching device, and an auxiliary power source whose positive terminal is connected to the source of the JFET switch and whose negative terminal is connected to the gate of the JFET switch, bypassing said gate-control circuit in one of the two positions of the switching device. The switching device is an electromagnetic or electric switch. The auxiliary power source may be a capacitor charged via a diode by a voltage powering the control device.
International patent application publication WO02/50897 A2 discloses an electronic power circuit comprising at least one power semiconductor whose control inputs are connected to a trigger device and comprising a power supply which, on the output side, is connected to terminals of the trigger device and, on the input side, is connected to a device to which a supply voltage is applied. A self-conducting power semiconductor is provided as the power semiconductor to economically reduce the forward power losses and switching losses of the electronic power circuit. In one embodiment the electronic power circuit is an inverter comprising input lines. Between these input lines a capacitor is connected in series with a diode. The voltage over this capacitor is used to power the power supply. The diode ensures that a charge loaded onto the capacitor is kept there in case of a break down of an input voltage applied between the input lines.
There still is a need for an inverter constructed of normally conductive semiconductor switches in which nevertheless a short circuit between the input lines is avoided even with just a low voltage being applied between the input lines.